The present invention relates to a signal transmitter, which is suitable for employment, for example, of keyless entry system for vehicles, and a method of operating the signal transmitter.
In a keyless entry system installed, for example, in an automobile, doors of an automobile are usually locked or unlocked with transmission of data by using radio signals. Here, the signal transmitter includes a voltage booster circuit for the stable transmission of radio signal. An output voltage of a small capacity battery such as a button battery or the like is boosted up to a predetermined voltage higher than such an output voltage. Thereby, the data is transmitted using the radio signal with such a boosted voltage Vb.
However, if the output voltage of the battery is lowered due to deterioration by aging of the battery, difference between the output voltage and the predetermined boosted voltage Vb increases. Thereby, since a large amount of power of battery is consumed when the output voltage is boosted by the voltage booster circuit, the output voltage of the battery which is a power supply voltage of a microcomputer momentarily becomes lower than the lower limit value of the operation voltage range of the microcomputer. As a result, it is likely that the microcomputer can no longer operate normally and operation life of the signal transmitter may be shortened.
It is therefore proposed to intermittently control the boosted voltage of the voltage booster circuit at fixed intervals by a microcomputer. Thus, the voltage booster circuit provides intermittently periods where the battery power is not consumed so that the output voltage of battery does not lower the lower limit value of the operation voltage range of the microcomputer. The boosted voltage gradually rises and reduction of the output voltage of the battery is reduced. However, it is insufficient to only intermittently control the boosted voltage of the voltage booster circuit at fixed intervals.
First, the voltage boosting characteristic of the voltage booster circuit is not uniform because of difference of specifications and fluctuation in manufacture. Therefore, the starting time of voltage boosting is different depending on the voltage booster circuit employed. Moreover, this starting time of voltage boosting changes depending on temperature. Therefore, difference arises in the degree of reduction of the output voltage of the battery during the voltage boosting operation depending on the voltage booster circuit employed.
This point will be explained in detail with reference to FIGS. 7A to 7C, which show variation in operation of different voltage booster circuits. It is assumed in these figures that a voltage boosting control signal Vbc is outputted to a voltage booster circuit from a microcomputer in order to instruct the voltage booster circuit to execute the voltage boosting operation. This voltage booster circuit boosts a battery voltage VB by oscillating operation during the period where the voltage boosting control signal Vbc becomes high level. During the low level period, this voltage booster circuit does not oscillate. The voltage boosting control signal Vbc is the same voltage boosting control signal Vbc in the predetermined period in such a case that the voltage boosting operation of the voltage booster circuit is controlled by the microcomputer.
Although not illustrated here, the voltage boosting control signal Vbc is a repetitive signal of a fixed interval and is outputted to the voltage booster circuit from the microcomputer. That is, the high level period and low level period of the voltage boosting control signal Vbc are fixed.
Here, FIG. 7A illustrates the change of the oscillation pulse signal Vosc and the boosted voltage Vb of the voltage booster circuit and reduction in the output voltage VB of the battery in such a case that the starting time Ts of voltage boosting of the voltage booster circuit is equal to the standard time Tss. FIG. 7B illustrates the change of the oscillation pulse signal Vosc and the boosted voltage Vb of the voltage booster circuit and reduction in the output voltage VB of the battery in such a case that the starting time Tse of voltage boosting of the voltage booster circuit is shorter than the standard time Tss. Moreover, FIG. 7C illustrates the change of the oscillation pulse signal Vosc and the boosted voltage Vb of the voltage booster circuit and reduction in the output voltage VB of the battery in such a case that the starting time Tsd of voltage boosting of the voltage booster circuit is longer than the standard time Tss.
Here, the starting time Ts of voltage boosting means the period until the voltage booster circuit starts the oscillating operation from the input of the voltage boosting control signal Vbc to the voltage booster circuit.
Under the condition that the voltage boosting control signal Vbc is in the high level in FIGS. 7A to 7C, if the starting time of voltage boosting of the voltage booster circuit is equal to the shorter time Tse, the number of oscillating pulses of the oscillation pulse signal Vosc is larger than the number of oscillation pulses of the oscillating pulse signal Vosc in the standard time Tss. Therefore, the boosting degree of the boosted voltage Vb is larger than the that of the boosted voltage Vb in the case where the starting time of voltage boosting is equal to the standard time Tss. However, the reduction degree (Δve) of the output voltage VB of the battery is larger than ΔV of the output voltage VB of the battery in the case where the starting time of voltage boosting is equal to the standard time Tss.
In addition, when the starting time of voltage boosting of the voltage booster circuit is equal to the longer time Tsd, the number of pulses of the oscillating pulse signal Vosc is less than the number of oscillating pulses of the oscillation pulse signal Vosc. Therefore, the boosting degree of the boosted voltage Vb is smaller than that of the boosted voltage Vb in the case where the starting time of voltage boosting is equal to the standard time Tss. However, the reduction degree (ΔVd) of the output voltage VB of the battery is also smaller than that of the output voltage VB of battery when the starting time of voltage boosting is equal to the standard time Tss.
Accordingly, the longer the starting time of voltage boosting is, the smaller the reduction degree of the output voltage VB of the battery becomes. Thereby, the output voltage VB of the battery does not readily become lower than the lower limit value of the operation voltage range of the microcomputer. On the contrary, a longer time is required until the output voltage rises up to the predetermined voltage. Meanwhile, the shorter the starting time of voltage boosting is, the larger the reduction degree of the output voltage VB of the battery becomes. Thereby, a longer time is not required until the voltage rises up to the predetermined value. On the contrary, the output voltage VB of the battery is readily lowered below the lower limit value of the operation voltage range of the microcomputer.
FIG. 6 shows a characteristic curve L identifying the relationship between the starting time Ts of voltage boosting and the boosted voltage Vb as the qualitative common characteristic of the voltage booster circuit. This characteristics is derived from the relationship between the starting time Ts of voltage boosting of each voltage booster circuit and the boosted voltage Vb. The characteristic curve L indicates that fluctuation exists in the characteristic of each voltage booster circuit but the starting time Ts of voltage boosting is rather short and almost does not change in the range where the boosted voltage Vb is low and the starting time Ts of voltage boosting rapidly becomes long when the boosted voltage Vb becomes high. It is thus understood that the starting time Ts of voltage boosting becomes longer when the boosted voltage Vb becomes near the predetermined voltage irrespective of the specification of the voltage booster circuit.
Therefore, when the high level period of the voltage boosting control signal Vbc is fixed to the constant value (Tc in FIG. 6), this high level period of the voltage boosting control signal Vbc matches with the starting time of voltage booting. Thus, the boosted voltage Vb saturates (L1 in FIG. 6) and does not rise and thereby the boosted output of the voltage booster circuit does not reach the predetermined boosted voltage Vb. This phenomenon arises at a lower boosted voltage Vb when the high level period of the voltage boosting control signal Vbc is shorter.
It is understood from the characteristic La shown in FIG. 6 that when the high level period of the voltage boosting control signal Vbc is set longer step by step considering that the starting time of voltage boosting becomes longer for the boosted voltage Vb on the characteristic curve L, saturation of the boosted voltage Vb can be prevented. It also becomes possible to prevent that the output voltage of deteriorated battery momentarily becomes lower than the lower limit value of the operation voltage range of the microcomputer.